Learning involves moving from a less adequate to a more adequate understanding of a task. Unfortunately, in many studies of learning, performance is described before and after the task has been mastered while little attention it paid to the transition between these states. The absence of focus on transition may reflect difficulties inherent in identifying the fleeting transitional state that occurs between the more stable endpoints of learning. Although many different operational definitions of the transitional period have been proposed, few have stood up to empirical verification. On the basis of empirical evidence, a model is proposed of transitions in learning which holds that the learner, when on the verge of making progress in a problem, entertains more than one hypothesis with respect to the problem and does so in a single response. Evidence for such a model is derived from the observation that children who are in a transitional state with respect to a problem (as evidenced by their readiness to profit from instruction in the problem) frequently produce two distinct hypotheses, one in gesture and a different one in speech, in a single explanation. Thus, the mismatch between the information conveyed in gesture and the information conveyed in speech distinguishes those who are on the threshold of learning from those who are not. However, children in transition may not be aware that they have considered a problem from more than one point of view, since one of their two hypotheses is encoded in gesture. Indeed, gesture may tap knowledge that the child cannot explicitly articulate yet nevertheless appears to understand, albeit implicitly. The purpose of the proposed studies is to explore the knowledge reflected uniquely in gesture and its role in the acquisition of concepts. The studies have three aims: (1) To test the hypothesis that spontaneous gesture, when it conveys information not encoded in speech, provides insight into the implicit information a child possesses. Study 1 employs a recognition paradigm, often used to tap implicit knowledge, to determine whether the knowledge a child demonstrates in gesture but not in speech is apparent in some other aspect of the child's behavior; children, ages 9 to 10, will participate in the study. (2) To test the hypothesis that spontaneous gesture reflects the leading edge of a child's competence, and thus expresses the areas in which the child will benefit from instruction. In Study 2, children, ages 9 to 10, will be given instruction tailored to the information they convey uniquely in their gestures in order to determine whether the children are particularly likely to benefit from such instruction. (3) To test the hypothesis that spontaneous gesture, occurring as it does in communicative contexts, provides an observable index of a child's implicit understanding of a problem, not only for experimenters trained in gesture coding, but also for untrained adults; if so, spontaneous gesture would provide a mechanism by which individuals who interact with a child can calibrate their input to the child s zone proximal development. Study 3 asks whether the spontaneous gestures children produce are interpretable by adults not trained in gesture coding. Study 4 asks whether untrained adults use the information they glean from gesture to alter the way they instruct a child.